1. Field of the Invention
The present invention relates to target units for use, for example, in a magnetron sputtering process.
2. Prior Art
FIGS. 1 and 2 depict a conventional target unit 1 of a rectangular shape which comprises a backing plate 2 of copper or stainless steel and a target 3 of a sputtering material securely fixed to the backing plate 2. The target 3 and the backing plate 2 are usually joined together by means of soldering with tin-lead alloy, indium, or the like.
The target unit 1 of the above construction is used in a magnetron sputtering apparatus as schematically illustrated in FIG. 3. The sputtering apparatus includes a vacuum chamber 4, a vacuum pump assembly 5 comprised of a diffusion pump 5A and a rotary pump 5B, a holder 6 for holding a substrate P, and a magnet assembly 7. As shown in FIG. 4, the magnet assembly 7 comprises a casing 7A and magnets 7B with a yoke 7C housed in the casing 7A, and the backing plate 2 of the target unit 1 is so fixed to the upper peripheral end of the casing 7A through bolts or the like as to hermetically seal the casing 7A. The casing 7A is supplied with cooling water which cools the target unit 1 from its back side. Thus, a magnetic field of an oblong shape is produced above the surface of the target 3 upon sputtering and causes plasma to concentrate on the surface of the target 3, thereby enhancing an efficiency of sputtering.
In the aforementioned target unit 1, however, only that portion A of the target 3 on which the magnetic field concentrates is mainly consumed, as shown in FIG. 4, and hence, the service life of the target 3 comes to an end with the sputtering material of the other portion unused.
The problem of this kind is often posed in target units for use in photomagnetic recording media, and in relation to this research has been actively carried out in recent years. More specifically, the target 3 of the target unit 1 for such an application is made of an alloy including a rare earth metal such as terbium (Tb), gadolinium (Gd) and dysprosium (Dy), and a transition metal such as iron (Fe) and cobalt (Co), and hence exhibits ferromagnetism. Therefore, when a recess as at A is formed in the target 3, a magnetic field such as to cause plasma to concentrate in the recess A is produced, so that the consumption of the bottom of the recess A is further promoted.
The target 3 which has ended its service life can be scrapped if the material is inexpensive, but since the rare earth metal used in making targets for photomagnetic recording media is expensive, the consumed target 3 must be removed from the target unit 1 and reclaimed into a new target.
In the conventional target unit 1, however, the target unit 1 must be entirely removed from the magnet assembly and heated to melt the solder, so that the reclamation of the target 3 is laborious and time-consuming, resulting in an increase in costs.